FIG. 1 is a perspective exploded view of a conventional optical head arrangement, which is shown in Japanese Patent Publication No. SHO 63-15661.
A laser diode 1 acting as a point light source emits a diverging laser beam that impinges on a condenser lens 2. The laser beam is collimated by the condenser lens 2 and, then, passes through a diffraction grating 3 to reach a polarizer prism 4.
The laser beam entering into the polarizer prism 4 has been polarized in a predetermined direction so that the beam can pass through the prism without changing its direction.
The beam that has passed through the polarizer prism 4 enters into a .lambda./4 plate (quarter-wave plate) 5 where linear polarization is converted to circular polarization. The beam then reaches a mirror 6 which deflects the beam by 90.degree. so that the beam impinges on an actuator 7.
The beam is projected through an objective lens 8 in the actuator 7 onto an information medium 12.
In the surface of the information medium 12, there are formed tracks with a spacing of several microns between adjacent tracks. Pits having a depth of .lambda./4 are arranged in the tracks, where .lambda. is the wavelength of the laser beam. The presence of the pits is detected by moving the optical head relative to the information medium in the direction in which the tracks extend, whereby signals are read out from the information medium 12.
When the information medium 12 is rotating, the spacing between the objective lens 8 and the information medium 12 varies due to variations in flatness of the surface of the information medium 12. In order to maintain this spacing constant, the position of the actuator 7 in the direction of the optical axis (hereinafter referred to as focus direction) is automatically controlled so that the laser beam is always focused to a spot having a diameter of about 1 micron on the surface of the information medium 12 in which the pits are formed.
The position of the actuator 7 is also controlled relative to the tracks in the medium 12 in a direction perpendicular to the direction of the length of the track and, hence, in the radial direction of the medium (hereinafter referred to as tracking direction) so that the laser beam spot does not go out of position with respect to a particular track being detected.
The position of the actuator 7 is also controlled in the direction along which the tracks extend. (Hereinafter, this direction is referred to as the jitter direction.)
Practically, the three-dimensional control of the position of the actuator 7, namely, the control in the focus, tracking and jitter directions, may be achieved in the following manner. For example, the control of the position of the actuator 7 in the focus direction may be carried out by moving the actuator 7 itself, and as for the control in the tracking and jitter directions, two oscillatory mirrors provided in the actuator 7 may be moved instead of moving the actuator 7, or use of such an oscillatory mirror may be combined with electrical processing. The actuator 7 is moved in the three directions so as to accommodate itself to error in flatness of the information medium 12 or to the eccentricity of the tracks, whereby the laser beam is always focused to a spot having a diameter of about 1 micron on a particular track being detected. Thus, stable recovery or recording of a signal from or into the information medium 12 can be achieved.
Since the direction of traveling of the laser beam reflected from the information medium 12 is reversed by 180.degree. relative to the incoming beam, it is circular-polarized with rotation in the opposite direction to the incoming beam. This reflected laser beams travels along the same optical path as the incoming beam to reach the .lambda./4 plate 5 where it is converted to a linear-polarized beam of which the direction is rotated .lambda./2 relative to the polarized beam emitted by the laser diode 1. The linear-polarized beam is deflected 90.degree. by the polarizer prism 4 to pass through a cylindrical lens 9, which is used to control the position of the actuator 7 in the focus direction. The beam which has passed through the lens 9 impinges onto a detector 10.
The respective optical elements disposed along the optical path extending from the laser diode 1 to the detector 10 are in their particular positions predetermined relative to the optical axis of the laser beam and are secured in position with a high precision with respect to a casing 11.
FIG. 2 illustrates how the above-described optical elements are mounted to the casing 11. The respective optical elements are secured to the casing 11 by means of screws 1-1, 1-2, . . . 9-1, 9-2. However, the polarizer prism 4 and the mirror 6 are secured in place by bonding adhesive to associated surface portions of the casing 11.
Various configurations for the casing 11 have been proposed. The above-described configuration is a reduced-thickness type in which various optical elements are arranged in a single plane that is parallel to the plane of the surface of the information medium 12. Another configuration is a vertical type in which various optical elements are arranged in a plane that is perpendicular to the plane of the surface of the information medium 12. One example is shown in Japanese Unexamined UM Publication No. SHO 61-132528.
In view of the purpose of reducing the thickness of optical information recording/reproducing apparatus, the reduced-thickness type casing described above may be preferable.
However, since a number of optical elements are mounted thereon, the area of the portions of the casing that are parallel to the plane of the surface of the information medium 12 is large, and, therefore, the casing of this type may suffer from mechanical resonance.
Now, mechanical resonance of the casing is considered. Generally, an optical head resonates at a certain frequency f.sub.1. Characteristics of an optical head are shown in FIGS. 3(A) and 3(B). FIG. 3(A) shows a gain characteristic curve, while FIG. 3(B) shows a phase characteristic curve. In the illustrated example, the gain is at a peak at a frequency of 7.3 KHz, and also the phase is inverted by 180.degree. at 7.3 KHz. Thus, it is seen that this point is a resonant frequency point. The presence of such a resonant point can pose a problem on servo control such as focus control, tracking control and jitter control stated previously.
In principle, when the gain at the resonant point exceeds the servo loop gain, it is no longer possible to provide the servo control, and, therefore, it becomes impossible to focus the laser beam to a spot of about 1 micron diameter on the surface of the information medium 12. As a result, proper recording and recovering of information cannot be done, which degrades the quality of information. In the worst case, no servo control can be achieved and, hence, recording and recovery of information cannot be done.
Of course, if the mechanical resonant frequency f.sub.1 is higher than a frequency band of interest for the servo control, there will be no problem.
However, in the reduced-thickness type of optical heads, such as the conventional optical head shown in FIG. 1, the resonant frequency f.sub.1 is still lower and the peak of the gain (Q) is still higher.
An optical information recording/reproducing system to be used as an peripheral unit for a computer is required to have an average access time that is short. Accordingly, the optical head has to be able to move at a high speed in the radial direction of an information medium. In order for the optical head to be able to move fast in the radial direction of the information medium, it has to be light in weight, which requires that the casing be made as lightweight as possible and that unnecessary portions be eliminated as much as possible. However, in such a structure, the problem relating to the mechanical resonance becomes more serious.
If the head tends to mechanically resonanate, it cannot move at a high speed even if it is made light-weight, and the servo control is adversely affected, as stated previously. That results in degradation of information quality or, in the worst case, failure of information recording or recovery.
As stated in the above, while the structure of the casing of the example described above is advantageously suitable for reducing the thickness of the casing, it is accompanied by the mechanical resonance problem.